Shield box and tuning mechanism for electron tubes



Ncv. 9, 1948. F. H. MccALL ET AL 2,453,148

SHIELD Box AND TUNING MECHANISN FOR ELEcTRoN TUBES Filed July 29 1 943 5 Sheets-Sheet 1 Nov. 9, 1948. 2,453,148

SHIELD Box AND TUNING MECHANISM FOR ELECTRON TUBES Filed July 29, 1945 F. H. MCCALL El'AL sheets-Sheet 2 m or. .S

INVENTOR FRED H. MCCALL BY TER DEVREOTES A TTORNE Nov. 9, 1948. F. H. MQCALL ErAl. I 2,453,148

SHIELD BOX AND TUNING MECHANISM FOR ELECTRON TUBES Filed July 29, 1943 v y :s sheet'sneet s v g FRED HMCCALL BY W PETER DEV oTEs ATTORNEY Patented Nov. 9, 1948 SHIELD BOX AND TUNING MECHANISM FOR ELECTRON TUBES Fred H. McCall, Neptune, and Peter Devreotes, Eatontown, N. J.

Application July 29, 1943, Serial No. 496,653

(Cl. Z50-36) (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) 16 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to us of any royalty thereon.

This invention relates to apparatus using electron tubes, and more particularly to a shield box and tuning mechanism therefor.

The primary object of our invention is to generally improve radio apparatus using electron tubes, and more specifically the shielding and tuning thereof.

An ultra high frequency oscillator tube of good performance is that known as the VTI-158. This tube is described in a co-pending application of Harold A. Zahl, Serial Number 473,556, filed January 25, i943. One object of our invention is to improve generally the shielding and operation of the said tube. Another object. of the invention is to improve the tuning of the cathodes of the said tube, and more especially to provide a cathode tuning unit which is disposed in a horizontal position for compactness; which will provide symmetrical tuning loops; and which may be made up as a rigid structure, rigidly supported at both ends, without necessitating the use of R. F. insulators across the tuning loop.

Other objects are to shield the tube; to shield the cathode tuner, and to shield the tube and tuner from one another.

A further object is to provide a shield box which may be readily and rapidly opened, and which when opened exposes the tube and tuner and their connections inside the shield box so fully and widely as to facilitate rapid change to a new tube when the old one has worn out.

To the accomplishment of the foregoing general objects, and other objects which will hereinafter appear, our invention resides in the shield box and cathode tuner elements and their relation one to the other as are hereinafter described in the following specification. The specification is accompanied by drawings in which:

Figure 1 is a partially sectioned elevation of a shield box embodying features of our invention;

Figure 2 is a plan view thereof;

Figure 3 is a front end elevation thereof, drawn to a reduced scale;

Figure 4 is a similar elevation showing how the box is opened;

Figure 5 is a side elevation with the box opened;

Figure 6 is a schematic wiring diagram explanatory of the wiring for filament heating;

Figures 7, 8 and 9 schematically show how the shield box may be used for either cathode, anode or grid keying;

Figure i0 is a partially sectioned plan View of the cathode tuner;

Figure 11 is a partially sectioned side elevation of the same;

Figure ll-A is a section taken in the plane of the line IIA-I IA of Figure 1l;

Figure 12 is explanatory of the method of making the lower part of the shield box;

Figure 13 is a section taken in the plane of the line I3-I3 of Figure 12;

Figure 14 is a side elevation of the lower part of the shield box;

Figure 15 is a section taken in the plane of the line iE-IS of Figure 14; and

Figure 16 is a plan View of the partition used in the shield box.

Referring to the drawing and more particularly to Figures 1 and 2, the shield box is a metal enclosure divided into an upper compartment I2 and a lower compartment I4 by means of a metal partition it. The ViP-158 tube, indicated in broken lines at I8, is housed in the upper compartment I2. A filament-tuning loop generally designated 20 is housed in the lower compartment I4. The shelf or partition I6 serves not only to support the tube I8 but also to shield the tube and the tuning loop 20 from each other. (This is in addition to the shielding of both the tube and the tuning loop from external fields.) The tube is cooled by means oi an air blower 22 driven by a motor 24 (Figure 2). The air-intake and the air-outlet o the blower are shown at 26 and 28 respectively. The outlet opens directly into the rear end of the shield box, thus blowing a stream of air directly around the tube I8. The air is discharged through screened windows. The main window is the window 3B (Figure 3), located in the upper compartment at the forward end of the box. There are also two windows 32 in each side wall or the box, these windows preferably being located in the lower compartment, as is best shown in Figures l and 5. The absence of side windows in the upper compartment insures the passage of air all the way through the upper compartment around the entire length of the tube. The windows in the lower compartment cause some of the air to flow downwardly around the glass stems projecting through the partition.

The bottom windows also aiiord convenient observation of the snorting bars of the cathode tuner. The windows are all screened with appropriate copper screening in order to complete the shielding of the apparatus.

In order to understand some of the features of casarse the present invention, it may be desirable to outline the arrangement of tube ld. rhis tube comprises tuned U-shaped circuits @il (Figure l) connected to the anodes, and additional tuned U- shaped circuits 31B therebeneath, connected to the grids. lThe grids project upwardly and are disposed within the anodes. Filamentary cathodes disposed within the grids, there being four cathodes, grids and anodes, two at each side of the tube. The anode leads 38 project upwardly through glass stems fill. Grid-circuit support wires l2 project downwardly through glass stems dil. e. pair of wires for each filament projects downwardly through each of four glass stems fit. The four filaments are connected in parallel for the heating current, but pairs of the ilaments are at opposite phase in respect to radio frequency voltage.

Referring now to Figure 6, the four iilaments are schematically represented at d8, all, and The leads of filament 43 are connected as shown to bars or straps of metal 5B, 5d. in practice these are disposed face to face with a thin sheetJ of mica therebetween, thereby insulating the bars one from another with respect to the low voltage filament heating current, but not with respect to radio frequency energy. The leads of element also are connected to the bars of 5t and The leads of filaments 52 and lill are connected to another pair of bars or straps of metal til, t2, which o are similarly disposed face to face with a thin sheet of mica therebetween. The inner bars 5%, 60 may be connected together at while the outer bars 5E, 62 are connected together at SS. It is thus possible to heat all four iilaments in parallel by means of low voltage connections 6B, ld.

The filaments 43, Sill operate in phase opposition or push-pull with respect to filaments 52, 5d. rThe push-pull related nlaments 5d, 52 are tuned by means of a loop or trombone adjusted by a slidable U-shaped spring or snorting bar l2. Similarly the laments 54 are tuned by means of a slidable spring or snorting bar M. These shorting bars 'l2 and 'lll are preferably simultaneously by means of screws 80, which are screwed into bar 52 but which are insulated from bar il? by appropriate washers. In the same way the bars Ell, 62 and mica film 'i8 are assembled by means of screws 82. 'Two of these screws may be used to hold soldering lugs M. The inner bars 58, Ell are iixedly secured to the metal block Sil by screws 86. The outer bars 55, 62 are xedly secured to metal block 56 by screws Sd. This provides a rigid tuning-bar structure devoid of R. F. insulators between opposite sides of the tuning loop. The use of metal spacing blocks Gil and G5 at both ends also facilitates sturdy support of the tuning loop within the shield box. in Figure ll it will be seen that these blocks are secured to plates SG which are fastened to the upper ends of insulators S2. The lower ends of the insulators are secured to the bottom Mld of the shield box. The insulators 92 may be moulded of ceramic material, and are designed to insulate the tuner for high voltage direct current, but are not sub"- i jected to a radio frequency load because they are located at E.. F. potential nodes.

Each of the adjustable shorting bars 'l2 and 'M comprises a metal block 94 carrying a pair of Phosphor bronze or equivalent springy Contact arms S6, which spring outwardly against the inner bars 58, 59. The ends of the springs preferably have small outwardly bent tabs to hold the arms against vertical displacement. These tabs must not project far enough to touch the outer bars 56, F52. The blocks Sil are carried by a threaded rod Idil having rightand left-hand threads H32, llll. The rod fill is made of a suitable insulation, such as Bakelite, and therefore does not short-circuit the filament heating supply at the end blocks 64, 66. The use of insulation for the rod lD also protects the operator against D. C. voltage when using the control knob iil, the latter being secured to the forward end of the rod at a point outside of the shield box, as is best shown in Figures 1 and 2. In order to increase the insulation for this purpose, the rod is preferably extended at the forward end of the shield boX and the projecting end is supported by a surrounding shielding can |08.

Reverting to Figures l0 and il, the rod 00 is supported against axial movement by means of appropriate threaded collars lill and lock nuts H2 screwed thereagainst. The collars il@ preferably have holes for a Spanner wrench or capstan bar. Rotation of the control knob l 05 causes simultaneous opposite movement of the threaded blocks 9d, thereby equally lengthening or shortening the lament tuning loops.

Clamp plates lill, H5, lid, and lill may be provided for detachable connection of the filament leads to the bars 5t, '58, 6l? and t2 respectively. These plates are L-shaped in section, as is best shown in Figure 11A, and are held in assembled relation by means of screws |22, which are threadedly received in the inner L-shaped plates, but which are insulated from the remainder of the structure, as will be clear from inspection of Figure 11A..

rIhe clamp plates l lll-20 are released by loosening the screws i222, whereupon one lead from filament Aid (Figure 6) is disposed between plate H4 (Figure 10) and bar :it at the left end of plate I l. The other lead from filament 48 is disposed between bar 53 and plate H6 at the left end of the plate. rhe top flanges of the plates are preferably notched as shown at i215 to nx the position of the filament leads. rIhe other` lament leads are clamped in position in a similar fashion at the other notches. Proper location of the lament leads is important because it affects the overall length of the -lament tuning loops. This is so because the tuning loop includes the vertical filament leads and also the horizontal portion of the tuning bars disposed between the point of attachment ci the leads and the shorting bars 94.

The glass envelope of the tube I8 is generally cylindrical. It is supported or cradled in position by means of four blocks E26 (Figures l, 5 and 16) which are fastened on top of partition I6. The upper ends of these blocks are properly sloped or concaved to mate with the cyllldTCal Surface of the tube. The tube is additionally supported against upward movement by means of four spring pressed blocks 23 best shown in Figures 1 and ll. These blocks are carried on the inner ends of spring arms i3d, the outer ends of which are held by blocks 32 fastened to the top of the shield box. The blocks l26--l28 which come into direct contact with the heated tube envelope are made of a suitable heat-resistant material. A good example is the refractory insulation known commercially as Mycalex.

The top wall |34 is cut away to pass the upwardly directed stems of the tube, and the resulting aperture or window is preferably finished by a rounded metal rod |36 (see Figure 2)', thereby providingr a smooth rounded finish to prevent high voltage breakdown to adjacent conductors connected to the anode leads 38.

The partition I6 beneath the tube is provided with three holes, best shown in Figure 16. The smaller outer holes |38 receive the downwardly directed stems 44 (Figure l) of the grid circuit support leads, while the large cent-er hole (Figure 16) receives four downwardly directed stems 46 (Figure 1) of the filament support leads.

The shield box is openable for tube replacement, and inasmuch as a change of tube may have to be made very rapidly as, for example, under field conditions in military use, the box is preferably made readily and rapidly openable and is so designed that when opened the tube and all of the internal connections are fully exposed. Referring to Figures 4 and 5, the top wall |34 and the side walls |42 are made of a single piece of sheet metal. The front and rear end walls |44 and the bottom wall |46 are also made of a single piece of sheet metal. When the box is changed from the closed condition shown in Figures 1, 2 and 3 to the open condition shown in Figures 4 and 5, the tube and the filament tuner and all of the internal connections are fully exposed. The top is shown adjacent the bottom to sho-w the relation therebetween, but the top may be removed altogether.

The detailed construction of the particular shield box here illustrated may be further explained by reference to Figures 13 and 15 of the drawing. The end walls |44 and the bottom wall |46 are formed of a single piece of sheet metal, which is preliminarily bent to channel shape as shown in Figure 13. Mitre notches |48 (Figure 12) are cut in the side walls so that the channel may be bent to the shape shown in Figure 14. The resulting mitred corners are brazed or welded, etc., at |50. The ends of the side wall may be cut away as indicated at |52 in Figure 12, thereby providing projecting pieces of metal which may be bent over as indicated at |54 in Figure 14, and fastened by brazing, welding or the like.

The shelf I6 is itself made of a piece of sheet metal bent to channel shape,as indicated in Figure 15. The width of the shelf is slightly less than the width. of the channel shown in Figures 12 and 13, so that the shelf will be received within the side flanges of the end walls |44. The ends of the shelf are turned downwardly as is indicated at |56, the side nanges of the shelf preferably being preliminarily notched as was explained at |52 in Figure 12. The down-turned ends of the shelf are preferably tapped to receive small screws |58 (Figures 1 and 3), which hold the shelf in position.

The upper part of the shield box fits slidably around the lower part shown in Figure 14. In order to clamp the same in position and to complete eifective electrical shielding at the vertical edges, the lower part of the box is preferably provided with four vertical clamping strips |60. (See Figures 1 through 5.) These strips may A be tightened or loosened by means of thumb 6 ed, brazed or otherwise secured to the inwardly flanged edge |66 of the shield box. The knurled knob is xed to the screw and acts as its head, the inside head |63 of the screw serving merely to prevent complete removal and loss of the screw. The side walls |42 of the shield box are a little shorter than the top wall (see Figure 5), so that they will be received between the vertical rows of screws |62. With this arrangement the upper part of the shield box is readily slid upwardly when the thumb screws |62 (and vertical clamp plates |60) are loosened, but the parts are securely fastened together lwhen the screws are tightened. The bottom edges also are preferably clamped against the bottom of the box. In Figure 5 it will be seen that the sides are notched at |68 to t around the bottom thumb screws |10 within the knurled heads thereof.

As so far described the external shielding is complete, but the partition IB may not adequately isolate the upper and lower compartments of the box. To help perfect the shielding between the upper and lower parts of the box, we provide spring strips |12 (see Figure 4) which extend horizontally (Figure 5) along the side walls 42 and are so disposed as to bear against the side flanges of the partition 6 when the box is closed. The spring strips |12 are preferably made of a highly-conductive resilient material, such as phosphor bronze. They do not interfere with the desired rapid opening or closing of the shield box.

Appropriate connections are provided to permit the oscillator tube to be connected for either cathode, anode or grid keying. Cathode keying is schematically illustrated in Figure 7. In this arrangement, the tuned anode leads 38 are grounded at |14 and a high minus D. C. potential is applied to the cathodes at |16. It will be understood that in practice the cathodes are not connected directly together but instead are connected to the filament tuning bars, as is shown in Figure 6. The plate and grid loops |18 and are disposed within the sealed glass envelope of the tube The lead 42 to the grid circuit |80 may be connected to a suitable bias resistor |82. The keying pulse is applied to the cathodes at the point |16.

Plate and grid keying arrangements are shown in Figures 8 and 9 respectively. In both cases the cathodes are grounded (through the cathode tuner), and a high positive plate potential is applied to the tuned anode leads, while a negative grid bias potential is applied to the grid circuit. In the case of Figure 8, however, the keying pulse is applied at the point |84, while in Figure 9 the keying pulse is applied at the point |86.

Referring now to Figure l, a suitable large porcelain insulator |88 is secured to the back end wall of the shield box. A pair of concentric conductors (mutually insulated for low voltage) pass through the insulator |88. Filament heating current may be applied to the lugs |90 and |92. The inner lug may also be used for the keying pulse, etc. The conductors are terminated within the box by lugs |94 and |96, which in turn are connected by short direct leads to the lugs 84 and 85 on the filament tuner. (These connections correspond to the connections 68 and 10 shown in Figure 6.) Grid bias potential may be applied to soldering lug |98 of conical porcelain lead in 200. The lug 202 within the box is readily connected by a short direct lead to the downwardly projecting wire 42 of the tube. The anode connections are, of course, made to the 7. upwardly projecting terminals. 38. of the tube.` Aground connection to the shield box-itself-may be made at the wing nut 204. With the connectionsvthus providedthe tubemay be'wired-for any. ofthe keying methods shown in Figures 7, 3 and 9).

It is believed that the constructionand operation of our improved shield box and iilament tuner, as well as the manyadvantages thereof, will be apparent from the foregoing detailed description. The shield boxis compact, relative to the size of the tube, It raises the efciencyof the tube and minimizes radiation therefrom. The particular tube illustrated oscillates at a frequency of, say 600 mc. At ultra high frequencies of this magnitude the problemy of efficient shielding isfcritical, and the dimensions of the shield box should be properly-selected toobtain reflection of the energy in correct phase.

The-.high frequency tube-here considered generates a small but appreciable quantity of X-rays. The shield box, if made of reasonably heavy gauge sheet metal, Sayles-inch thick, protects the operator from injuryv from the relatively soft X-rays.

Thev filament tuner is a rigid structure which does not require radio frequency insulation between its sides. Two equal loops are provided, and are symmetrically aranged to provide identical tuning for the pairs of vcathodes even though four cathodes are used. The desired equal length ofrloop would-be readily'obtainable by using a single loop disposed vertically, but in such case a cumbersome structure of substantial height would result. The present structure is made compact largely, by reason of the horizontal disposition of the tuner. With a` single loop disposed: horizontally, however,l the loop would be longer for one pair of cathodes than for the other. Moreover, a single tuning loop would require radio frequency insulation to support the open endl of the loop, The present'arrangement overcomes all of these difliculties `and provides a sturdy structure which is both rigid and efficient and which may be solid-ly supported at both ends within the shield box.. The shield box not only shields the-tube and the tuning loop,`but also shields them from one another. The shield box is readily opened, and when open exposes the tube and all of itsconnections, thus'facilitating tube change.

It-wil'l bey apparent that while we have shown` and describedour invention in-a preferred form,v manyichanges andy modifications may be made in the structure disclosed Without departing from the'spirit of theinvention, as soughtto be defined in the following claims.

We claim:

1.' A metal shield box for an ultra high irequency oscillator tube, said box being divided into two compartments by a horizontal partition, the top'and'sides of said box being made of sheet metal into a unitary structure, the ends,l bottom and partition `of said box being made of sheet metalinto a second unitary structure, the top and sides being upwardly slidable to4 completely open the Ishield box and thereby expose the tube for replacement, the said sides of said box having horizontally disposed spring strips so positioned asto bear against the side edges of the aforesaid partition in order to effectively complete the shielding action of the partition, said box having spaced blocks to support the bottom portion of the tube; the removable top of said-'box having yieldable means to bear against-the to-p portion of the tube when the box isclosed.

2. Ar metal shield boxl for an ultra high frequency oscillator tube requiring an external cathode tuner, said box comprising an upper ccmpartment enclosing and shielding the tube and a lower compartment enclosing and shielding the cathode tuner, the partition between said compartments being a metallic shield for shielding the tuner and tube from one another, the top and sides of said box being made of sheet metal into a unitary structure, the ends, bottom and partition of said box being made of sheet metal into a second unitary structure, the partition being secured between the ends, and the top and sidesbeing upwardly slidable to completely open the shield box and thereby expose. the tube for replacement.

3. A metal shield box for an ultra high frequency oscillator tube requiring an external cathode tuner, said box comprising an upper compartment enclosing and shielding the tube and a lower compartment enclosing and shielding the cathode tuner, the partition between said compartments being -a metallic shield for shielding the tuner andl tube from one another, the top and sides of said box being unitary structure made of sheet metal, the ends, bottom. and partition of said box being a second unitary structure made of sheet metal, the partition beingsecured between the ends, the topand sides being upwardly slidable to completely open the shield box and thereby expose the tube in the upper compartment for replacement,

said partition having spaced blocks to supporttheV envelope of the tube, and the top of said box having a spring means to bear downwardly against the envelope of the tube when the box is closed.

4. A metal shield box for an ultra high frequency oscillator tube requiring an external cathode tuner, said box comprising an upper compartment adapted to enclose and shield the tube and a lower compartment adapted to enclose and shield the cathode tuner, a partition between said compartments being a metallic shield adapted to shield the tuner and tube from one another, the top and sides of said `b-ox being a unitary structure made of sheet metal, the ends, bottom and partition of said box being a second unitary structure made of sheet metal, the partition being secured between the ends,V the top and sides being upwardly slidable to. completely open the shield box and thereby expose the tube in the upper compartment for replacement, and the sides of said box having horizontally disposed spring strips so positioned as to bear against the side edges of the aforesaid partition in order to eifectively complete the shielding action of the partition.

5. .A metal shield box for an ultra high-frequency oscillator tube, the top and sides of said box being a unitary structure made of sheet metal, the ends and bo tom of said box being a second unitary structure made of sheet metal, and a blower connected to one end of the shield box, said box having one or more windows for. freey flow of cooling air, said windows being screened with fine mesh metal screening for shielding purposes, the top and sides being upwardly slidable tocompletely open the shield box and thereby allow a tube to be installed or removed.

6.. A metal shield box-for an ultra high frequency oscillator tube requiring an external cathode tuner, said box comprising an upper compartment adapted to enclose and shield said `tube and a lower compartment adapted to enclose and shield said cathode tuner, a partition `between said compartments being a metallic shield adapted to shield the tuner and tube from one another, a blower connected to one end of the shield box at the tube compartment, said tube compartment having one or more windows for free iiow of cooling air, said windows being screened with iine mesh metal screening for shielding purposes, the top and sides of said box being a unitary structure made of sheet metal, the ends, bottozn and partition of said boX being a second unitary structure made of sheet metal, the partition being secured between the ends, the top and sides being upwardly slidable to completely open the shield box and thereby allow a tube to be installed in or removed from said partition having spaced blocks to support the envelope of the tube, and the top of said box having spring means to bear yieldably downwardly against the envelope of the tube.

7. A metal shield box for an ultra high frequency oscillator tube requiring an external cathode tuner, said box comprising an upper compartment adapted to enclose and shield said tube and a lower compartment adapted to enclose and shield said cathode tuner, a partition between said compartments being a metallic shield adapted to shield the tuner and tube from one another, a blower connected to one end of the shield box at the tube compartment, said tube compartment having one or more windows for free flow of cooling air, said windows being screened with iine mesh metal screening for shielding purposes, the top and sides of said boX being a unitary structure made of sheet metal,

the ends, bottom and partition of said box being i a second unitary structure made of sheet metal, the partition being secured between the ends, the top and sides being upwardly slidable to completely open the shield box and thereby allow a tube to be installed in or removed from the upper compartment, and the sides of said box having horizontally disposed spring strips so positioned as to bear against the side edges of the aforesaid partition in order to effectively complete the shielding action of the partition.

8. A tunable cathode loop for a push pull o.,- cillator tube, said loop comprising two spaced parallel bars, each of said bars including two strips of metal insulated with respect to each other for direct currents but not, for radio frequency currents, whereby Lhe heater leads of one cathode may be connected to the two strips respectively of one bar and the heater leads of the other cathode may be connected to the two strips respectively of the other bar, a snorting contact slidable along the bars, and means for adjusting the position of the shorting contact along the bars.

9. A tunable cathode loop for a push pull oscillator tube, said loop comprising two spaced parallel bars, each of said bars including two strips of metal separated by thin insulation, whereby the heater leads of one cathode may be connected to the two strips respectively of one bar and the heater leads of the other cathode may be connected to the two strips respectively of the other bar, a slider disposed between the bars and slidable along the inner strips, and screw mechanism for adjusting the position of the slider along the bars.

l0. A tunable cathode loop for a push pull oscillator tube, said loop comprising two spaced parallel bars, each of said bars including two strips of metal separated by thin insulation, whereby the heater leads of one cathode may 10 be connected to the two strips respectively of one bar and the heater leads of the other cathode may be connected to the two strips respectively oi the other bar, a U-shaped spring or slider disposed between the bars and slidable along the inner strips, a screw made of insulation and threadedly received by said slider for adjusting i the position of the slider along the bars, and a metal block between the inner strips at one end of the strips.

ll. A tunable cathode loop for an oscillator tube having two pairs of cathodes, said loop comprising two spaced bars, each of said bars including two strips of `metal insulated from each other with respect to direct current but not radio frequency currents, the cathode leads of one pair of cathodes being connected to the two strips of one bar, the cathode leads of the other pair of cathodes being connected to the two strips of the other bar, a slider disposed betweenithe bars and slidable along the inside strips near one end of the bars, a slider similarly slidable near the other end of the bars, and means to simultaneously equally move the sliders in opposite directions, so that the tuning loop remains symmetrical with respect to all of the cathodes.

12. A tunable cathode loop for an oscillator tube having two pairs of cathodes, said loop comprising two spaced bars, each of said bars including two strips of metal insulated from each other with respect to direct currents but not radio frequency currents, a metal block connecting the inner strips at one end of the bars, a second metal block connecting the outer strips at the other end of the bars, the cathode leads of one pair of cathodes being connected to the two strips of one bar, the cathode leads of the other pair of cathodes being connected to the two strips of the other bar, a slider disposed between the bars i and slidable along the inside strips near one end of the bars, a slider similarly slidable near the other end of the bars, and insulated mechanism for simultaneously equally moving said sliders in opposite directions, so that the tuning loop remains symmetrical with respect to `all of the cathodes.

13. A tunable cathode loop for an oscillator tube having four cathodes, said loop comprising two spaced parallel bars, each of said bars comprising including two strips of metal separated by thin insulation, a metal block connecting the inner strips at one end of the bars, a metal block -connecting the outer strips at the other end of the bars, insulators supporting the blocks respectively, the cathode leads of one pair of cathodes being connected to the two strips of one bar, the cathode leads of the other pair of cathodes being -connected to the two strips of the other bar, a U-shaped spring or slider disposed between the bars and slidable along the inside strips near one end of the bars, a second U -shaped spring or slider similarly slidable near the other end of the bars, and an insulation screw passing through the blocks and freely rotatable relative thereto and i having rghtand left-hand threads7 one of said threads receiving one of said sliders and the other of said threads receiving the other of said sliders, whereby both sliders may be simultaneously equally moved in opposite directions, so that the tuning loop remains symmetrical with respect to all of the cathodes.

14. A shield box and tunable cathode loop for an oscillator tube including two cathodes, said loop including two spaced parallel bars, each of said bars including two strips of metal insulated 'fromeach other with respect to direct currents vbut not radio frequency currents, the cathode `leads of one cathode being connected to the two strips of one bar, the cathode leads of the other cathode being connected to the two strips of the other bar, a slider disposed between the bars'and slidable along the bars, an insulation screw threadedlyreceiving the slider, and a metal shield box shielding the tube and the cathode tuning loop, said metal shield lbox having a partition disposed "between the tube and the loop, one end of said threaded rod projecting out of one end of the shield box and having a control knob for rotation thereof, said end of the shield box having a shield- 'ing can projecting therefrom and surrounding the projecting end of the rod in order to space the knob a substantial distance from the loop so as "to protect the operator from any high direct current potential on the loop.

' 15. A shield box and tunable cathode loop for an oscillator tube having four cathodes, said loop including two spaced parallel bars, each of said bars including two strips of metal insulated from each other with respect to direct currents but not radio frequency currents, a-metal block connecting .the inner strips at one endof the bars,a second metal block connecting the outer `strips at'the other .end of the bars, the cathode leads of "one pair of cathodes being connected to the two strips of one bar, the cathode leads of the other pair'of cathodes being connected to the two strips of the other bar, a slider disposed between the bars and slidable along the inside strips near one end of 'the bars, a second slider similarly slidable near `the other end of the bars, and an insulation screw lhaving-.right and left hand threads, one of'said threads receiving one of said sliders and the other of sai'dthreads receiving the other of said i'sliders, whereby both sliders may besimultane- 'ously' equally moved in opposite directions, so that `the `tuning loop remains symmetrical with re- 'spect to `all ofthe cathodes, and a metal shield *box shielding the tube and the cathode tuning `loop,"said metal shield box having a partition disposed between the tube and the loop, and one end of said threaded rod projecting out of one end ofthe shield box to allow rotation of the rod.

vI6. A shield box and tunable cathode loop for anvoscillator tube having four cathodes, said loop I-including two spaced parallel bars,each of said i I'bars including two strips of metal separated Aby Athin insulation, a metal block connecting the :to Number 132 inner stripsat one end of the bars, a metal block lconnecting the outer strips at the other end of the bars, insulators supporting the blocks respectively within the box, the cathode leads of one pair of cathodes being connected to the two strips of one bar, the cathode leads of the other pair of cathodes being connected to the two strips of the other bar, a U-shaped spring or slider disposed between the bars and slidable along the inside strips near one end of the bars, a second U-shaped spring or slider similarly slidable'near the other end of the bars, an insulation screw passing through said blocks and having right and left hand threads, one of said threads receiving one of said sliders and the other of said threads 'receiving the other of said sliders, whereby both sliders may be simultaneously equally moved in opposite directions, `so that the tuning loop lremains symmetrical with respect to all of the cathodes, and a metal shield box shielding-the tube and the cathode tuning loop, said metal shield-box having a'shielding partition disposed between the tube and the loop', one end of said threaded rod projecting out of one end ofthe r shield box-and having a'control knob for rotation REFERENCES 'CITED The following 'references are of record in the Aille of'this patent:

UNITED STATES PATENTS Name Date Hoxie June 28, 1921 Brower Dec. 3, 1929 Snow Jan. 31, 1933 Clavier Sept. 26, 1933 Mitchell Sept. 13, 1938 Henderson July 18, 1939 Usselman Sept. 17, 1940 Tinus Oct. 6, 1942 FOREGN PATENTS Country Date Great Britain Feb. v6, 1930 Number Certiicate of Correction Patent No. 2,453,148. November 9, 1948. FRED H. MCCALL ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 7, line 6, after the numeral 9 strike out the parenthesis; line 15, for ma read Mc.; column 9, line 15, claim 6, alter the Word from insert the Words the fupper compartment; column 10, line 15, claim 11, for current read currents; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofce.

Signed and sealed this 12th day of April, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,453,148. November 9, 1948. FRED H. MCCALL ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 7, line 6, after the numeral 9 strike out the parenthesis; line 15, for mc. read Mc.; column 9, line 15, claim 6, after the Word from insert the Words the upper compartment; column 10, line 15, claim 11, for current read currents;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 12th day of April, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uommzssoner of Patents. 

